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Pockets of water may cause tremors

Tuesday, 6 January 2009 - 1:45pm

THE CANADIAN PRESS

VANCOUVER—Researchers at the University of British Columbia are offering an explanation to recurring tremors underneath Vancouver Island: pockets of water deep under the Earth that act as a lubricant between tectonic plates.
Vancouver Island lies on a megathrust fault—an area of massive and infrequent earthquakes.

The Cascadia megathrust fault runs along North America’s western coast from northern Vancouver Island to northern California, and is the boundary between two of the planet’s tectonic plates.
An area on the fault line roughly 35 km under Vancouver Island has experienced regular “slips,” accompanied by small tremors, every 14 months or so.
Megathrust fault lines in the region where episodic tremors occur are structurally weak and prone to slip and slide, but scientists have been unable to explain why.
In a study published in the latest edition of the journal “Nature,” UBC researchers say water trapped in a portion of the fault area escapes periodically after pressure build-up, which lubricates the tectonic plates and causes them to slip.
“Scientists have offered different theories, but this is the first detailed glimpse at the geological mechanics beneath the island,” said the article’s lead author Pascal Audet, who conducted the study as a PhD student at UBC’s department of earth and ocean sciences.
The research was carried out using seismometers spread over Vancouver Island.
The meters read teleseismic waves emitted by the fault line to determine density and pressure differences.
The only plausible explanation for the readings and differences, according to the report, was the presence of water underground.
“While scientists are still a long way away from being able to predict earthquakes, this study brings us one step closer toward understanding the physical state of the megathrust fault and the earthquake cycle as a whole,” Audet said.
Now scientists would like to expand the research to include studies of the stronger tremors that regularly occur in the waters off Vancouver Island.
The last one was on Christmas Day, when a magnitude 5.9 earthquake struck 187 km west of Port Hardy.
“Additional sensors on the island, or expanding the sensor array into the waters west of Vancouver Island, could help researchers determine whether fault properties change over time, and where changes are most significant along the fault line,” said UBC Dean of Science Simon Peacock, who also worked on the study.